1. Field of the Invention
The present invention relates to systems for slurry-based abrasive finishing and polishing of substrates; more particularly, to such systems employing magnetorheological fluids (MRF) and magnets adjacent to a spherical carrier wheel for magnetically stiffening the fluid in a work zone on the wheel; and most particularly, to an improved system wherein the stiffening magnets are disposed within the wheel itself.
2. Discussion of the Related Art
Use of magnetically-stiffened magnetorheological fluids for abrasive finishing and polishing of substrates is well known. Such fluids, containing magnetically-soft abrasive particles dispersed in a liquid carrier, exhibit magnetically-induced plastic behavior in the presence of a magnetic field. The apparent viscosity of the fluid can be magnetically increased by many orders of magnitude, such that the consistency of the fluid changes from being nearly watery to being a very stiff paste. When such a paste is directed appropriately against a substrate surface to be shaped or polished, for example, an optical element, a very high level of finishing quality, accuracy, and control can be achieved.
U.S. Pat. No. 5,951,369 issued Sep. 14, 1999 to Kordonski et al., the disclosure of which is hereby incorporated by reference, discloses methods, fluids, and apparatus for deterministic magnetorheological finishing of substrates. This patent is referred to herein as xe2x80x9c""369.xe2x80x9d
In a typical magnetorheological finishing system, such as is disclosed in the ""369 patent, a work surface comprises a vertically-oriented wheel having an axially-wide rim which is undercut symmetrically about a hub. Specially shaped magnetic pole pieces are extended toward opposite sides of the wheel under the undercut rim to provide a magnetic work zone on the surface of the wheel, preferably at about the top-dead-center position. The surface of the wheel is preferably an equatorial section of a sphere.
Mounted above the work zone is a substrate receiver, such as a chuck, for extending a substrate to be finished into the work zone. The chuck is programmably manipulable in a plurality of modes of motion and is preferably controlled by a programmable controller or a computer.
Magnetorheological fluid is extruded in a non-magnetized state from a shaping nozzle as a ribbon onto the work surface of the wheel, which carries it into the work zone where it becomes magnetized to a pasty consistency. In the work zone, the pasty MRF does abrasive work, known as magnetorheological polishing or finishing, on the substrate. Exiting the work zone, the concentrated fluid on the wheel becomes non-magnetized again and is scraped from the wheel work surface for recirculation and reuse.
Fluid delivery to, and recovery from, the wheel is managed by a closed fluid delivery system such as is disclosed in the ""369 reference. MRF is withdrawn from the scraper by a suction pump and sent to a tank where its temperature is measured and adjusted to aim. Recirculation from the tank to the nozzle, and hence through the work zone, at a specified flow rate is accomplished by setting the speed of rotation of a pressurizing pump, typically a peristaltic pump. Because the peristaltic pump exhibits a pulsating flow, a pulsation dampener is required downstream of the pump.
The rate of flow of MRF supplied to the work zone is highly controlled. An inline flowmeter is provided in the fluid recirculation system and is connected via a controller to regulate the rotational speed of the pump.
A capillary viscometer is disposed in the fluid delivery system at the exit thereof onto the wheel surface. Output signals from the flowmeter and the viscometer are inputted to an algorithm in a computer which calculates the apparent viscosity of MRF being delivered to the wheel and controls the rate of replenishment of carrier fluid to the recirculating MRF in a mixing chamber ahead of the viscometer, to adjust the apparent viscosity to aim.
The prior art MRF finishing system just described is unsuited to two finishing requirements which have recently emerged.
First, because the magnet pole pieces are extended under the edge of the wheel from outside the envelope of the sphere from which the wheel is derived, on substantially a tangent to the spherical surface, the prior art system cannot finish large concave objects such as large lenses having a radius of curvature on the order of the radius of the wheel, because of steric interference of the pole pieces.
Second, because the pole pieces extend radially over a comparatively small central angle of the wheel, the prior art system is useful for finishing of workpieces only when they are disposed at or near the top dead center position of the carrier wheel and thus is limited to finishing substrates which may be mounted and manipulated by an overhead chuck.
It is a principal object of the invention to provide a system for magnetorheological finishing of concave substrates wherein the radius of the concavity is comparable to the radius of the carrier wheel.
It is a further object of the invention to provide a system for magnetorheological finishing of substrates wherein the finishing may be carried out at any desired angular orientation of the work zone on the carrier wheel.
It is a still further object of the invention to provide a system for magnetorheological finishing of large substrates wherein the substrate is positioned on a controllable bed, the carrier wheel is positioned over the substrate, and a work zone is provided at the bottom dead center position on the carrier wheel and may be moved over the substrate.
Briefly described, an improved system for magnetorheological finishing of a substrate in accordance with the invention comprises a vertically oriented carrier wheel having a horizontal axis. The carrier wheel is preferably an equatorial section of a sphere, such that the carrier surface is spherical. The wheel is generally bowl-shaped, comprising a circular plate connected to rotary drive means and supporting the spherical surface which extends laterally from the plate. An electromagnet having planar north and south pole pieces is disposed within the wheel, within the envelope of the sphere and preferably within the envelope of the spherical section defined by the wheel. The magnets extend over a central wheel angle of about 120xc2x0 such that magnetorheological fluid is maintained in a partially stiffened state well ahead of and well beyond the work zone. A magnetic scraper removes the MRF from the wheel as the stiffening is relaxed and returns it to a conventional fluid delivery system for conditioning and re-extrusion onto the wheel. The placement of the magnets within the wheel provides unencumbered space on either side of the carrier surface such that large concave substrates, which must extend beyond the edges of the wheel, may be accommodated for finishing. The angular extent of the magnets causes the MRF to be retained on the wheel over an extended central angle thereof, permitting orientation and finishing in a work zone at the bottom dead center position of the wheel.